The antigenic requirements for the stimulation of cytotoxic thymus-derived lymphocyte (T killer cell) lines will be studied. We will explore which products of the various H-2 region genes are responsible for T killer cell proliferation and activation and whether these are different from the ones serving as target antigens in the cytotoxic reaction. T killer cell lines specific for syngeneic tumor cells will be studied with regard to the question of what antigenic determinants they recognize. Attempts will be made to characterize and possibly isolate the T killer cell receptor using various biochemical approaches and selected T killer cell lines as a source of cells.